This application claims the priority of German patent 198 28 816.6-21, filed Jun. 27, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a hydraulic power steering system for motor vehicles which is suitable for a steer-by-wire operation.
A power steering system of this type is known, for example, from German Patent Document DE 195 41 752 A1. The known power steering system has a steering handle which is constructed, for example, as a manual steering wheel and which is manually operated by a driver of the vehicle for steering the vehicle. In addition, the known power steering system has a hydraulic servo motor which, with respect to its drive, is coupled with steerable vehicle wheels and correspondingly operates these for steering the vehicle. In addition, the power steering system has a servo valve which operates the servo motor and is constructed as a rotary slide valve arrangement whose mutually rotatable control parts together are forced by means of a spring system preferably constructed as a torsion spring into a normal position relative to one another. Furthermore, the known power steering system has an actuator for operating the servo valve, in which case the actuator is directly or indirectly coupled with respect to the drive with the control parts of the rotary slide valve arrangement for their rotational adjustment with respect to one another. In addition, a desired steering angle value generator, which is operated by means of he steering handle, and an actual steering angle value generator, which is operated by means of the steerable vehicle wheels, as well as an automatic control and control arrangement are provided, in which case, for steering the vehicle, this automatic control and control arrangement operates the actuator as a function of a comparison of the desired and the actual steering angles.
In the case of servo valves constructed as rotary slide valve arrangements, manufacturing tolerances can lead to hydraulic asymmetries of the servo valve. Likewise, frictional losses, for example, in seals, may lead to hysteresis effects and thus to dead times in the servo valve, which impair a precise position control of the servo motor and thus an exact tracking stability of the vehicle.
The present invention relates to the problem of improving the automatic control performance in the case of a power steering system of the initially mentioned type.
According to the invention, this problem has been solved by providing a power steering system for a motor vehicle, comprising: a manual steering handle; a hydraulic servomotor which operates steerable vehicle wheels; a servo valve which operates the servo motor, said servo valve having two control parts which are rotatable relative to each other; a spring system which biases said control parts into a normal position relative to each other; an actuator which is directly or indirectly coupled with the control parts of the servo valve, said actuator being operable to adjust a relative angular position between the control parts; an angle sensor which is coupled directly or indirectly with the control parts of the servo valve, said angle sensor detecting the relative angular position between the control parts; a desired steering angle value sensor which senses a desired steering angle based on a position of the steering handle; an actual steering angle value sensor which senses an actual steering angle based on a position of the steerable vehicle wheels; and an automatic control arrangement which operates the actuator as a function of a comparison of the desired steering angle and the actual steering angle, and as a function of the relative angular position between the control parts.
According to the invention, this problem has been solved by providing a control system for a hydraulic power steering system for a motor vehicle having a manual steering handle, a hydraulic servomotor which operates steerable vehicle wheels, a servo valve which operates the servo motor, said servo valve having two control parts which are rotatable relative to each other, a spring system which biases said control parts into a normal position relative to each other; an actuator which is directly or indirectly coupled with the control parts of the servo valve, said actuator being operable to adjust a relative angular position between the control parts, said control system comprising: an angle sensor which is coupled directly or indirectly with the control parts of the servo valve, said angle sensor detecting the relative angular position between the control parts; a desired steering angle value sensor which senses a desired steering angle based on a position of the steering handle; an actual steering angle value sensor which senses an actual steering angle based on a position of the steerable vehicle wheels; and an automatic control arrangement which operates the actuator as a function of a comparison of the desired steering angle and the actual steering angle, and as a function of the relative angular position between the control parts.
According to the invention, this problem has been solved by providing a method of controlling a hydraulic power steering system for a motor vehicle having a manual steering handle, a hydraulic servomotor which operates steerable vehicle wheels, a servo valve which operates the servo motor, said servo valve having two control parts which are rotatable relative to each other, a spring system which biases said control parts into a normal position relative to each other; an actuator which is directly or indirectly coupled with the control parts of the servo valve, said actuator being operable to adjust a relative angular position between the control parts, said method comprising: detecting the relative angular position between the control parts; sensing a desired steering angle based on a position of the steering handle; sensing an actual steering angle based on a position of the steerable vehicle wheels; and operating the actuator as a function of a comparison of the desired steering angle and the actual steering angle, and as a function of the relative angular position between the control parts.
In this case, the invention is based on the general idea of detecting via an angle sensor the control path of the servo valve and using it as an additional control value for improving the automatic control. For example, the actuating power which can be generated by the servo motor depends on the control path of the servo valve which, as a rule, is constructed as a proportional control valve so that, as the result of the detection of the control path, the actuating power on the servo motor can be better apportioned. A desired steering angle change can then also be implemented with lower control expenditures and particularly more rapidly if the automatic control and control arrangement of the power steering system always adjusts the suitable control path at the servo valve.
Furthermore, in the case of servo valves which are equipped with a so-called "open center", special advantages exist when lateral forces are controlled Which act upon the vehicle wheels from the outside. In the case of a servo valve with an open center, all connections of the servo valve communicate with one another when the servo valve assumes its normal position. A servo valve with an open center has the important advantage that a pressure source of the hydraulic system--which preferably is a hydraulic medium pump, can continuously supply hydraulic pressure. In the normal position of the servo valve, the not required hydraulic pressure is relaxed by way of its open center into a corresponding reservoir. If the servo motor is to be operated, the required hydraulic pressure is immediately available.
However, in the normal position of the servo valve, lateral wheel forces may by way of the open center of the servo valve cause an adjustment of the servo motor and thus a steering angle change. In order to counteract this, a suitable countercontrol force must be generated by a corresponding operation of the servo valve on the servo motor. Since, in the case of the power steering system according to the invention, a targeted, apportioned servo valve adjustment can be carried out in a particularly simple manner, the power steering system according to the invention can appropriately react to lateral wheel forces, without any large automatic control expenditures. The reason is that, as soon as an interfering force acts upon the steerable vehicle wheels, as a function of the interfering force, an apportioned counterforce can be applied by way of a correspondingly targeted servo valve control.
In addition to the reduced automatic control requirements as a whole, the overall steering system is stabilized, in which case particularly natural oscillations cannot build up in the control circuit.
Corresponding to an advantageous embodiment of the power steering system according to the invention, pressure gauges may be provided which are used for determining a differential pressure between two hydraulic connections of the hydraulic servo motor, the hydraulic connections being acted upon by hydraulic pressure by the servo valve for operating the servo motor. By means of the differential pressure existing between the hydraulic connections of the servo motor, a quantity can be determined in a simple manner which correlates with the actuating power of the servo motor. As a result, it is also possible to adjust the actuating power required for the adjustment of a desired steering angle or to determine an interfering force acting upon the steered vehicle wheels.
Furthermore, corresponding to a further development of the power steering system according to the invention, an assignment can be carried out between the differential pressures at the hydraulic connections of the servo motor and the relative angular positions of the control parts of the rotary slide valve arrangement with respect to the another. Likewise, an assignment can be carried out between the relative angular positions of the control parts of the rotary slide valve arrangement and the valve flow rate and thus of the adjusting speed of the servo motor and of the steered wheels. In addition, devices for determining the actual viscosity of the hydraulic medium can also be provided by use of which the valve flow rate can be determined more precisely. The determination of the hydraulic medium viscosity can be carried out, for example, by measuring the temperature. By means of such assignments, which each form a characteristic curve of the servo valve, it is possible to balance or adjust tolerance-caused deviations of the hydraulic symmetry of the servo valve electronically by a corresponding calibration of the assignment. The power steering system thereby becomes independent of manufacturing tolerances and can therefore ensure a precise control of the servo motor in both steering directions and thus an exact tracking stability of the vehicle.
In particular, it is possible to further develop the automatic control arrangement and control arrangement such that the above-mentioned assignment between the relative angular position of the control parts of the rotary slide valve arrangement and the differential pressure between the hydraulic connections of the servo motor is constantly updated. As a result, on the one hand, wear phenomena of the hydraulic system can be compensated constantly; and, on the other hand, particularly a diagnosis of the hydraulic system can be carried out with respect to its operability, for example, in that a comparison with defined threshold values is constantly carried out.
It is understood that the above-mentioned characteristics and the characteristics which will be explained in the following can not only be used in the respective indicated combination but also in other combinations or alone without leaving the scope of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.